The present invention relates to an organic electroluminescent device constructed by forming at least an emitting layer using an organic material between a hole injection electrode and an electron injection electrode, and is characterized in that stable luminance can be obtained for a long time, and high luminance can be obtained at a low voltage particularly in an organic electroluminescent device having an emitting layer constructed by doping a dopant into a host material.
In recent years, the needs of flat panel display devices the consumption of electric power and the size of which are smaller than those of a CRT (cathode-ray Tube) which has been heretofore generally employed have been increased as information equipments are diversified, for example. An electroluminescent device has been paid attention to as one of the flat panel display devices.
The electroluminescent device is roughly divided into an inorganic electroluminescent device and an organic electroluminescent device depending on a used material.
The inorganic electroluminescent device is so adapted that a high electric field is generally applied on a luminance portion, and electrons are accelerated within the high electric field to collide with a luminescence center, whereby the luminescence center is excited to emit light. On the other hand, the organic electroluminescent device is so adapted that electrons and holes are respectively injected into a luminescent portion from an electron injection electrode and a hole injection electrode, the electrons and the holes thus injected are recombined with each other in a luminescence center to bring an organic molecule into its excited state, and the organic molecule emits fluorescence when it is returned from the excited state to its ground state.
In the case of the inorganic electroluminescent device, a high voltage of 100 to 200 volts is required as its driving voltage because the high electric field is applied as described above. On the other hand, the organic electroluminescent derive can be driven at a low voltage of approximately 5 to 20 volts.
In the case of the organic electroluminescent device, a luminescent device emitting light in a suitable color can be obtained by selecting a fluorescent material that is a luminescent material. It is expected that the organic electroluminescent device can be also utilized as a multi-color or full-color display device, for example. Further, it is considered that the organic electroluminescent device is utilized as a back light of a liquid crystal display device or the like because it can emit light at a low voltage.
In recent years, various studies have been conducted on such an organic electroluminescent device.
In such an organic electroluminescent device, an emitting layer and a carrier transport layer which is constituted by a hole transport layer for transporting holes to the emitting layer and an electron transport layer for transporting electrons thereto are generally provided between a hole injection electrode and an electron injection electrode. Specifically, used as the structure thereof are a three-layer structure referred to as a DH structure obtained by laminating a hole transport layer, an emitting layer and an electron transport layer between a hole injection electrode and an electron injection electrode, a two-layer structure referred to as an SH-A structure obtained by laminating a hole transport layer and an emitting layer abundant in electron transport properties between a hole injection electrode and an electron injection electrode, and a two-layer structure referred to as an SH-B structure obtained by laminating an emitting layer abundant in hole transporting properties and an electron transport layer between a hole injection electrode and an electron injection electrode.
In a conventional organic electroluminescent device, however, it is generally difficult to obtain as an organic material used for its emitting layer high-purity one by sublimation and purification, for example, and the stability of the organic material to heat or the like is not sufficient. When the organic electroluminescent device emits light for a long time, therefore, some problems occur. For example, the organic material used for the emitting layer is crystallized to form pinholes due to heat or the like at the time of the light emission, so that uniform and sufficient luminance cannot be obtained for a long time.
In recent years, in order to increase luminous efficiency in the emitting layer in the organic electroluminescent device, a dopant having a high quantum efficiency of fluorescent has been doped into a host material constituting the emitting layer.
In thus doping the dopant into the host material constituting the emitting layer, however, there are some problems. For example, unless a combination of the host material and the dopant is suitably selected, sufficient luminance cannot be obtained.
An object of the present invention is to solve the above-mentioned problems in an organic electroluminescent device constructed by forming at least an emitting layer using an organic material between a hole injection electrode and an electron injection electrode.
That is, an object of the present invention is to make it possible to obtain stable luminance for a long time by preventing pinholes from being formed by crystallization of an organic material used for an emitting layer due to heat or the like at the time of emitting light as in the conventional example.
Another object of the present invention is to make it possible to obtain, in an organic electroluminescent device having an emitting layer obtained by doping a dopant into a host material, high luminance by sufficient light emission of a dopant doped into the host material in the emitting layer.
The present invention is so adapted, in an organic electroluminescent device constructed by providing at least an emitting layer using an organic material between a hole injection electrode and an electron injection electrode, that a dopant selected from coronene, rubicene, pyrene, benzpyrene, chrysene, ovalene, fluorocyclene, picene, triphenylene, aceanthrene, fluoranthene, acenaphthene, acenaphthylene, benzanthracene, naphthafluorene, naphthafluorenone, naphthapyrene, anthraquinone, rubrene peroxide, pentacene quinone, perylene quinone, naphthacene quinone, benzofluorenone, benzofluorene, anthrafluorene, benzperylene, benzpentacene, bispyrenyl propane, tetramethyl naphthacene, dibenzanthracene, pyrene quinone, perylene, and fluoracene, or their derivatives is used, and the difference between the highest occupied molecular orbital in the host material and the highest occupied molecular orbital in the dopant is in a range from xe2x88x920.3 eV to +0.3 eV.
When the above-mentioned dopant is thus doped into the host material, molphology in air is hardly changed, so that the film stability of the emitting layer is improved, and the organic material used for the emitting layer is prevented from being crystallized due to heat or the like at the time of emitting light. Therefore, stable luminance is obtained for a long time.
When the difference between the highest occupied molecular orbital in the host material and the highest occupied molecular orbital in the dopant is set in a range from xe2x88x920.3 eV to +0.3 eV, excitation energy is efficiently moved from the host material to the dopant. Therefore, luminous efficiency in the organic electroluminescent device is improved, so that high luminance is obtained. Particularly when the difference between the highest occupied molecular orbital in the host material and the highest occupied molecular orbital in the dopant is in a range from xe2x88x920.1 eV to +0.1 eV, the excitation energy is more efficiently moved from the host material to the dopant. Therefore, the luminous efficiency is further improved, so that higher luminance is obtained.
The above-mentioned dopant is low in molecular polarity, is easily sublimated, and has high heat resistance. Therefore, a high-purity dopant is easily obtained by sublimation and purification. When the high-purity dopant which is thus obtained by the sublimation and purification is doped into the emitting layer, uniform and higher luminance is obtained for a long time.